Roller thrust bearing

ABSTRACT

There is provided a roller thrust bearing providing reduced differential slip, free of significant friction and wear, and providing increased longevity against flaking. The present invention provides a thrust bearing in the form of a roller thrust bearing including at least two radially arranged rows of rollers and a cage holding the rollers, the rollers including at least one roller crowned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to roller thrust bearingsincluding needle roller thrust bearings and particularly to thrustbearings arranged in rotative portions of electronic components ofautomobile transmissions (manual and automatic), transfers orcompressors for automobile air conditioners and the like to supportthrust loads exerted on the rotative portions.

2. Description of the Background Art

With reference to FIGS. 6A and 6B, a roller thrust bearing 1 includes aplurality of circumferentially, radially equally arranged rollers 2, acage 3 formed in a circular ring and holding the plurality of rollers 2rotatably, and outer and inner rings 4 and 5 serving as a roller bearingring sandwiching and thus supporting the plurality of rollers 2 on theiropposite sides. Outer and inner rings 4 and 5 are each formed in acircle formed of a metal plate having a sufficient hardness. Outer ring4 has a circular ring surface 6 of its own and inner ring 5 has acircular ring surface of its own.

A roller thrust bearing is a bearing having a variety of advantages. Forexample it can have a simple structure and still provide high loadcapacity and high rigidity. Furthermore it can be structured without theaforementioned roller bearing ring and formed of a cage and a rolleralone. In that case, a shaft, a housing or the like that has acounterpart rolling contact surface to the roller is provided withroughness, hardness or any other similar function comparable to thebearing's ring surface, and using the shaft, the housing or the like asa ring surface allows the bearing to dispense with a roller bearing ringand thus be compact. In particular, a roller thrust bearing using aneedle roller, or a needle roller thrust bearing, can be decreased inheight, as seen in cross section, and thus suitable for compactmechanical designs.

A main application of a needle roller thrust bearing, a typical exampleroller thrust bearings, is a compressor of an automobile airconditioner. It includes a variety of types. For example, FIG. 7 shows adouble-inclined-plate compressor in which a 2-side inclined plate 9secured to an input rotary shaft 8 allows a piston 10 to reciprocate. Asanother example, FIG. 8 shows a single-inclined-plate compressor inwhich a 1-side inclined plate 12 secured to an input rotary shaft 11allows a piston 14 to reciprocate via a rod 13. As still anotherexample, FIG. 9 shows a variable-capacity, single-inclined-platecompressor in which an inclined plate 16 secured variably in angle to aninput rotary shaft 15 allows a piston 18 to reciprocate via a rod 17.The above types all have a rotative portion incorporating a rollerbearing.

Note that the FIGS. 7-9 compressors do not necessarily belong toconventional art. They should be considered an apparatus which willfurther be improved in future for example through further development ofa roller thrust bearing contemplated by the present invention. In otherwords, they are an apparatus having its internal components beingimproved in structure, material and the like.

In a typical bearing, such as ball bearings, a differential slip isintroduced between a rolling element and a roller bearing ring. Thedifferential slip of such bearings basically depends on a difference incircumferential velocity in a contact surface of the rolling element andthe roller bearing ring. More specifically, a ball bearing provides apoint contact and hence a small contact area. As such, within a contactsurface their difference in circumferential velocity and hence theirdifferential slip would be small.

In contrast, in a roller thrust bearing, as basically structured, arolling element in the form of a cylindrical roller is arranged on aroller bearing ring having a planar ring surface, the roller and theroller bearing ring contact each other in a line, and the bearing'scenter of rotation matches the roller's center of revolution. In thatcase, the circumferential velocity on a rolling contact surface of theroller is the same velocity, whereas the roller bearing ring contactingthe roller as it rotates provides a circumferential velocity increasing,as seen radially outward, away from the bearing's center of rotation (inproportion to the roller bearing ring's radius of rotation). Thereforethe roller and roller bearing ring's difference in circumferentialvelocity has a maximal value at the roller's opposite ends. In theory,only on a pitch circle of the bearing a no-slip rolling movement isprovided. From a point on a pitch circle of the roller toward theopposite ends of the roller, the difference in circumferential velocitybetween the roller and the roller bearing ring increases, and thedifferential slip increases, in proportion to the roller's length.

As aforementioned, differential slip internal to a roller thrust bearingis greater than those of bearings of different types. As such, adifferential slip of the roller and the roller bearing ringdisadvantageously causes generation of a stress at the roller's edgethat is introduced between the roller bearing ring and the roller, whichcauses flaking of an edge of a rolling and running portion of the rollerbearing ring that starts at a surface thereof.

The above disadvantage has conventionally been addressed by a bearinghaving a roller reduced in length to reduce relative slippabiliy of theroller's end surface or a bearing having two rollers arranged in eachpocket of the cage, i.e., in multiple rows. Furthermore, thedisadvantageous stress or load on the roller's end overcome for exampleby using a crowned roller (Japanese Patent Laying-open No. 9-14131).

There is a demand for a roller thrust bearing reduced in size to saveenergy and space and have a reduce weight (a reduced friction loss).Accordingly, the bearing in use encounters an increasingly severecondition in terms of load capacity. As aforementioned, in a rollerthrust bearing a differential slip of a roller and a roller bearing ringincreases friction loss and wear that are caused at a contact surface.Furthermore the contact surface's oil film formation is impaired, and atthe roller's edge an edge stress between the roller bearing ring and theroller itself is readily introduced, which causes flaking of an edge ofa rolling and running portion of the roller bearing ring that starts ata surface thereof. The differential slip may be reduced simply byreducing the roller in length. This, however, reduces the roller'scontact area and thus provides increased contact pressure. Contactpressure at the contact surface thus increases, which results inimpaired oil film formation and the exfoliation (flaking) for exampledisadvantageously occurs at the roller's radically outer surface.

A compressor uses a bearing having an interior with the bearing'slubricant mixed with a coolant. Furthermore, as the compressor contractsand expands, the lubricant is liquefied and vaporized repeatedly. Thelubricant is thus disadvantageously reduced in amount. As such, a worseoil film is formed than when a typical machine operating lubricant isused, and the bearing flakes earlier than expected.

Furthermore to address global warming and other similar environmentalissues a compressor for an air conditioner uses HCFC134a or othersimilar alternative flon as its coolant. These alternative flons areconsidered as being poorer in self-lubricity than conventionally usedcoolants. With the alternative flon dissolved and thus mixed, thelubricant for example has a reduced kinematic viscosity, and the bearinghas a significantly impaired oil film formation. As such, the bearingwould have a rolling element, a roller bearing ring and the likeflaking, wearing or having a surface similarly damaged, resulting inreduced longevity.

This may be handled by improving the lubricant. However, because of itschemistry with the coolant, the lubricant can only be selected from alimited range and a significantly enhanced ability to form an oil filmcannot be expected. Increasing the amount of the lubricant in thecoolant to enhance lubricity decreases the amount of the coolant andthus impairs the compressor's cooling ability.

As used in an air conditioner's compressor, a needle roller thrustbearing receives a thrust load offset from the center of rotation androtates rapidly at no less than approximately 8,000 rpm. Such a rotationrate and a load are also severer conditions, increasing a defectindicating damage of a surface of the bearing that is attributed to adifferential slip.

SUMMARY OF THE INVENTION

In view of the above the present invention contemplates a roller thrustbearing reducing a differential slip, free of significant friction andwear, and providing excellent flaking resistance.

The present invention provides a roller thrust bearing including atleast two radially arranged rows of rollers and a cage holding therollers, the rollers including at least one crowned roller.

The plurality of rows of rollers can effectively reduce a differentialslip. Furthermore, the crowned roller can have a reduced effectivelength to synergistically, effectively reduce the differential slip. Inaddition, the crowned geometry can effectively alleviate a load that isintroduced at an edge. Simultaneously, the bearing's torque can also bereduced.

Furthermore the cage preferably has a pocket each holding a singleroller so that the roller can be held in steady manner to prevent theroller's skew. Furthermore, friction torque attributed to the roller'sskew can also be reduced.

Furthermore a radially outer row can have less crowned rollers than theradially innermost row. The radially outer rollers can thus have alarger effective length to reduce contact surface pressure and thusprevent flaking of the radially outer rollers and a roller bearing ringradially outer than the radially outer rollers.

Furthermore, at least one row radially outer than the radially innermostrow is preferably formed of straight rollers. As well as the radiallyouter rollers less crowned than radially inner rollers, as describedabove, the radially outer, straight rollers can similarly have a largereffective length to reduce contact surface pressure and thus preventflaking of the radially outer rollers and a roller bearing ring radiallyouter than the radially outer rollers.

Furthermore, at least one row radially outer than the radially innermostrow may be formed of rollers larger in length than the radiallyinnermost row to reduce a relative slip of the radially inner rollersand increase a load capacity of the radially outer rollers to preventflaking of the radially outer rollers and a roller bearing ring radiallyouter than the radially outer rollers.

Furthermore the roller can have at least one end surface sphericallyshaped to prevent rollers from having their end surfaces significantlyinterfering with each other and the rollers' end surfaces and the cage'spockets from significantly interfering with each other and thus reducethe rollers' skew. Furthermore, friction torque attributed to therollers' skew can also be reduced.

Furthermore, at least one row radially outer than the radially innermostrow may have rollers no less in number than the radially innermost rowto increase a load capacity of the radially outer rollers to preventflaking of the radially outer rollers and a roller bearing ring radiallyouter than the radially outer rollers.

Furthermore the roller can be a needle roller. A bearing with the needleroller also has an effect similar to that described above.

The above-described roller thrust bearing can be incorporated in acompressor for use. In the compressor, despite space-saving required anda peculiar condition for lubrication including a coolant, as has beendescribed previously, the roller thrust bearing can be free ofsignificantly impaired oil film formation.

The present invention provides another roller thrust bearing including acrowned roller and a roller bearing ring contacting the roller, theroller bearing ring containing no less than 10,000 to less than 40,000grains/mm² of carbide at least to a depth of 0.1 mm, as measured from asurface layer thereof, the grain having a diameter size of no less than0.6 μm.

The roller that is crowned can eliminate a differential slip at an edgeto reduce a load. In roller bearing ring the carbide acts as aresistance to deformation to enhance the strength of the material(s) ofthe surface layer. Furthermore, if between the roller and the rollerbearing ring a differential slip, poor lubrication or the like isintroduced and the roller bearing ring has a surface generating heat,plastic flow can be prevented.

In the roller bearing ring at at least the surface layer to the depth of0.1 mm the carbide having a grain of 0.6 μm or larger in size can beprecipitated such that 10,000 or more of such grains exist in eachunitary area of 1 mm² to allow the carbide to act as a resistance toplastic deformation of the surface layer. To obtain a higher level of aneffect of the resistance, no less than 14,000 grains/mm² are desirablyprecipitated. More than 40,000 grains/mm² reduce flaking resistance.Note that the grain size is an average grain size with respect todirection if the carbide is not spherical.

The density of the carbide of the surface layer of the roller bearingring, as described above, can be obtained for example as follows: aSCM415 material is processed. For carburization, 890° C. by one hour isset and a furnace internally is set to have an internal atmosphere witha carbon potential of 1.5 to 2.0%. For quenching, 840° C. by 0.5 hour isset and a furnace is set to have an internal atmosphere with a carbonpotential of 1.0 to 2.0%. Thereafter, the material is oil-quenched toprovide the above density. The present invention is applicable to othersteel than carburized steel, such as bearing steel, carbon steel and thelike.

The density may be measured using an electronic microscope in a direct,thin film observation, an observation of an extraction replica, or a2-stage replica observation, or it may be measured by observing apolished, etched surface of a sample via a scanning electronicmicroscope.

The crowned roller can eliminate a differential slip that is introducedat an edge and reduce a load and the roller bearing ring with the abovecarbide added to its surface layer can mainly enhance a resistance toplastic deformation. Combining the crowned roller and the roller bearingring having a surface layer with a high density of carbide distributedtherein, can provide a remarkable resistance to surface-flaking thatsignificantly exceeds an effect achieved by a conventional needle thrustbearing with these requirements provided separately thereto and thenadded together. In other words, resistance to surface-flaking cansignificantly be enhanced by synergism of the roller that is crowned andthe high density of carbide distributed in the surface layer of theroller bearing ring. Furthermore, in the roller bearing ring, the rollerand the like, wearability is significantly reduced.

The above, another roller thrust bearing can include at least two rowsof rollers including at least one crowned roller.

The plurality of rows of rollers allow a differential slip to bedistributed to small, multiple rows. Furthermore the crowned roller cancontribute to a reduced load of an edge. As a result, a significantlyincreased longevity can be obtained against surface-flaking.Furthermore, friction torque can also be reduced and for example anautomobile's air conditioner can be operated at a reduced power cost.

The carbide in the roller bearing ring at the at least surface layer tothe depth of 0.1 mm may have a distribution precipitated in a range ofno less than 0.8% to less than 10% per unit area.

This can enhance both the roller bearing ring's anti-wearability andlongevity against flaking. The carbide of the above areal percentage isobtained for example as follows: a SCM415 material is processed. Forcarburization, 890° C. by one hour is set and a furnace is set to havean internal atmosphere with a carbon potential of 1.5 to 2.0%. Forquenching, 840° C. by 0.5 hour is set and a furnace is set to have aninternal atmosphere with a carbon potential of 1.0 to 2.0%. Thereafter,the material is oil-quenched to provide the above density. The presentinvention is also applicable to other steel than carburized steel ofSCM415, such as bearing steel, carbon steel and the like.

Preferably the above bearing has at least two rows of rollers, wherein aradially outer row has rollers less crowned than the radially innermostrow or free of crowning.

The less crowned roller does not significantly reduce a rolling contactportion, so that a load capacity of the radially outer rollers can beenhanced and a load contact pressure of each of the radially outerrollers can be reduced. As such, the radially outer rollers and a rollerbearing ring rotating and moving with the radially outer rollers can befree of flaking.

The bearing can have a cage holding the roller, the cage having a pocketarranged to accommodate-no less than two rollers with an axial directionextending radially.

The cage thus structured can be processed at a reduced cost and a skewper roller can be reduced.

Furthermore, the bearing may have a cage holding the roller, the cagehaving at least two rows of pockets each accommodating a single roller.

This can reduce a skew of a roller due to a support of the cage.Furthermore, it can also reduce friction torque. For example if it isused in an inclined-plate compressor for an automobile air conditionerthe former allows the latter to be operated at a reduced power cost.

Furthermore, the bearing may have at least two rows of rollers, whereina radially outer row has rollers no less in length than a radially innerrow.

The radially inner rollers can be free of significant differential slipand the radially outer rollers can have a larger rolling contact portionthan the radially inner rollers to provide enhanced load capacity.

The roller may have an end surface curved to protrude outward.

The geometry as above can prevent rollers from having their end surfacesinterfering with each other and the rollers' end surfaces and the cage'spockets from interfering with each other. It can also reduce therollers' skew and friction torque.

The above roller thrust bearing may have at least two rows of rollers,wherein a radially outer row has rollers no less in number than aradially inner row.

This can increase a load capacity of the radially outer rollers.

The roller bearing ring may have a surface having a Rockwell C scalehardness (HRC) of no less than 58.

The roller bearing ring having the above hardness can reduce a plasticflow of an edge portion of a rolling and running trace to be smallerthan a conventional roller bearing ring and also enhanceanti-wearability. For less than HRC 58, anti-wearability isinsufficiently enhanced, substantially equivalent to that of aconventional roller bearing ring.

In accordance with the present invention, in an inclined-platecompressor having an inclined plate secured to and rotated around a mainshaft to reciprocate a piston, a roller thrust bearing is provided at aportion contacting the inclined plate to receive a thrust loadintroduced as the main shaft rotates and the piston reciprocates. Thebearing includes a crowned roller and a roller bearing ring contactingthe roller, the roller bearing ring containing no less than 10,000 toless than 40,000 grains/mm² of carbide at least to a depth of 0.1 mm, asmeasured from a surface layer thereof, the grain having a diameter sizeof no less than 0.6 μm.

In the roller thrust bearing arranged in contact with an inclined platethere is introduced a differential slip between a roller bearing ringand a roller. This differential slip negatively affects a formation ofan oil film of a contact surface and impairs a lubricant in lubricity.In addition for example for a roller thrust bearing for aninclined-plate compressor for an automobile air conditioner, asaforementioned, a lubricant is thinned in use for example with a coolantof the air conditioner. This results in further impaired lubricity.Consequently, a rolling contact surface generates heat and a surfacereadily flakes or has similar damage. However, combining the crownedroller and the roller bearing ring having a surface layer with a highdensity of carbide distributed therein, can provide a remarkablyexcellent resistance to surface-flaking for example despite a conditionfor lubrication with a coolant of an automobile air conditioner and alubricant mixed together. This resistance to surface-flaking can obtaina remarkably improved, synergetic effect that significantly exceeds aneffect achieved by adding together effects of providing the roller, theroller bearing ring and the like with the above requirements separately.

The foregoing and other objects, features, aspects and advantages of thepresent invention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1A shows a roller of a roller thrust bearing of an embodiment ofthe present invention;

FIG. 1B shows another roller of a roller thrust bearing of an embodimentof the present invention;

FIG. 1C shows another roller used in a roller thrust bearing of anembodiment of the present invention together with the FIG. 1A rollerand/or the FIG. 1B roller;

FIG. 2A is a plan view of an upper half of a roller thrust bearing of anembodiment of the present invention (a first embodiment);

FIG. 2B is a plan view of an upper half of another roller thrust bearingof an embodiment of the present invention (the first embodiment);

FIG. 2C is a plan view of an upper half of still another roller thrustbearing of an embodiment of the present invention (the firstembodiment);

FIG. 3A is a plan view of a needle roller thrust bearing if anembodiment of the present invention with a resin mold cage (a secondembodiment);

FIG. 3B is an enlarged view of a portion A of FIG. 3B (the secondembodiment);

FIG. 3C is a cross section of FIG. 3B taken along a line IIIC-IIIC (thesecond embodiment);

FIG. 4A is a plan view of a needle roller thrust bearing of anembodiment of the present invention with a box-like, iron plate pressedcage (the second embodiment);

FIG. 4B is a cross section of FIG. 4B taken along a line IVB-IVB (thesecond embodiment);

FIG. 4C is an enlarged view of a pocket of FIG. 4 (the secondembodiment);

FIG. 5A is a longitudinal cross section of a needle roller thrustbearing of the present invention in a third embodiment;

FIG. 5B is a plan view of an upper half of the rollers and cage of FIG.5A;

FIG. 6A is a longitudinal cross section of a conventional needle rollerthrust bearing;

FIG. 6B is a plan view of an upper half of the rollers and cage of FIG.6A;

FIG. 7 is a longitudinal cross section of a double-inclined-platecompressor for an air conditioner;

FIG. 8 is a longitudinal cross section of a single-inclined-platecompressor for an air conditioner; and

FIG. 9 is a longitudinal cross section of a variable-capacity,single-inclined-plate compressor for an air conditioner

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention in an embodiment provides a needle thrust bearingincorporated for use in a prescribed rotative portion of a compressorfor an automobile air conditioner shown in FIGS. 6A-8. Specifically, byway of example, for the FIG. 7 double inclined plate type, a needleroller bearing with a cage 19 and a needle roller thrust bearing 20 areused. For the FIG. 8 single inclined plate type, a shell needle rollerbearing 21 and a needle roller thrust bearing 22 are used. For the FIG.9 variable-capacity, single inclined plate type, a needle roller bearingwith a cage 23 and a needle roller thrust bearing 24 are used. Theneedle roller thrust bearing, having an advantage such as smallness inheight, as seen in cross section, is thus used in a compressorparticularly in applications requiring space-saving.

As described above, the FIGS. 7-9 compressors of automobile airconditioners should be considered an apparatus which will further beimproved in future for example through further development of the rollerthrust bearing contemplated by the present invention. In other words,they are an apparatus having internal components to be further improvedin structure, material and the like.

(1) Geometry of Roller

FIG. 1A shows a geometry of a roller 2 a crowned 25. Roller 2 a has alength L, with opposite ends crowned by a length L₁. The crowned rollercan reduce a load on an edge (or reduce edge stress) and the rollerhaving a reduced effective length (a straight portion L₂) can reducedifferential slip.

FIG. 1B shows a roller having opposite end surfaces rounded. Thisstructure can reduce edge stress that is caused by contact of aninternal surface of a pocket of the cage and the roller's end surface inthe pocket's plane, prevent rollers from having their end surfacesinterfering with each other and the rollers' end surfaces and the cage'spockets from interfering with each other, and thus reduce the rollers'skew. Furthermore, a friction torque attributed to the roller's skew canalso be reduced. The aforementioned round geometry is typically ±50% ofa roller diameter Da.

In contrast to FIG. 1A, FIG. 1C shows a non-crowned roller, i.e., astraight roller. The non-crowned roller can have a straight potion withan increased length. It can reduce contact surface pressure in acondition free of a tilted rolling and running surface, in particular.Note that in the present invention the straight roller is arranged in atleast one row radially outer than the innermost row. These rollers aretypically formed of bearing steel having a surface hardness of HRC 60 to65.

(2) Arrangement of Roller

FIGS. 2A to 2C are each a plan view of a half of a roller thrust bearingused in a first embodiment of the present invention describedhereinafter. FIG. 2A shows a roller thrust bearing with a cage having asingle row of pockets each accommodating rollers 2 a crowned at oppositeends and arranged radially in two rows. Configuration is used in firstand second embodiment for sample 4 of an example of the presentinvention, as will be described hereinafter. FIG. 2B shows a rollerthrust bearing with a cage having two rows of pockets each holding asingle crowned roller. This configuration is used in the secondembodiment for sample 5 of an example of the present invention. FIG. 2Cshows a roller thrust bearing with a radially outer, at least one row ofrollers equal to or larger in number than the radially innermostrollers. This can increase the load capacity of the radially outerrollers to be larger than that of the radially innermost rollers. Thisconfiguration is used in the second embodiment for sample 10 of anexample of the present invention.

(3) Cage

FIGS. 3A-3C and 4A-4C show cages in an embodiment of the presentinvention. FIGS. 3A-3C shows one form (sample 4) used in the secondembodiment of the present invention. It uses a resin molded cage formedby injection-molding and a needle roller. In the present form, cage 3,inject-molded, can be provided with a pocket having a more complicatedgeometry. As shown in FIG. 3B, each pocket holds two (or two rows of)rollers and it has a protrusion at its center to prevent the roller fromdisplacing. As shown in FIG. 3C, the protrusion allows cage 3 to holdroller 2.

FIGS. 4A-4C shows another form of the second embodiment using a cagepressed formed by an iron plate in the form of a box and a needleroller. The present form is equivalent to that shown in FIGS. 3A-3C andalso corresponds to sample 4. The cage of the present form can bepress-formed inexpensively. As shown in FIG. 4C, two members 3 a and 3 bpress-formed by an iron plate clamp two rollers 2 in one pocket and thusassemble the form of a box. Note that the pocket has a width smallerthan the roller's diameter so that cage 3 can hold roller 2.

If the roller thrust bearings of FIGS. 3A-3C and 4A-4C use a rollerhaving a diameter Da and a length L, Da≦5 mm and 1≦L/Da≦10 arepreferably defined. More specifically, for L/Da≦1, the roller is reducedin length and hence effective length. The roller's contact area isreduced and an increased contact surface pressure is thus provided. Inthe contact surface, contact surface pressure is increased, whichresults in an impaired oil film formation and the exfoliation forexample disadvantageously occurs at the roller's radially outer surface.

For 10≦L/Da, the roller is increased in length and hence effectivelength and a differential slip of the roller and a ring surfaceincreases. At the roller's edge an edge stress between the rollerbearing ring and the roller itself is readily introduced, which causesdisadvantageous flaking of an edge of a rolling and running portion ofthe roller bearing ring that starts at a surface thereof.

First Embodiment

A longevity test was conducted using the above roller thrust bearing toverify that the present roller thrust bearing can contribute to areduced differential slip of the roller's end surface and a reducedstress concentration in a vicinity of the roller's end, and a reducedfriction loss and wear internal to the bearing to provide an increasedlongevity of an edge of a rolling and running portion against flaking.

The tested bearing had a roller having a diameter of 3 mm, a rollerbearing ring having an inner diameter of 65 mm and an outer diameter of85 mm. The test was conducted at a temperature of 60-80° C. with a loadof 1,000 kgf at 500 rpm with a spindle oil VG2 (oil film parameter,lambda: 0.101) applied as a lubricant.

The test provided a result, as shown in Table 1, together with that of acomparative example, a needle roller thrust bearing having a standardroller and a standard cage. Longevity was represented by a 10% longevityof 10 test bearings. Note that the roller had a length of 7.8 mm for asingle row and 3.8 mm by two for two rows and it was crowned (ordropped) by 5 to 15 micrometers. Longevity was defined as the time whenany of the members of the bearing being tested flaked.

As understood from Table 1, if comparative example sample 1 (a standardbearing) had a longevity of 1, then sample 2, having two rows ofrollers, had a longevity 2.5 times sample 1, and sample 3, havingcrowned rollers (in a single row), had a longevity 4.9 times sample 1,whereas sample 4, having two rows of crowned rollers, as provided in thepresent invention, had a longevity 7.5 times sample 1 and thussignificantly larger than samples 1 and 2, with its rollers completelyfree of flaking, as observed after it was tested. Furthermore in thetest a motor consumed a smallest current of 3.9 A for sample 4, whichindicates that the bearing was of a low torque and it can be understoodthat friction loss internal to the bearing was reduced.

It is thus apparent that a roller thrust bearing having two or more rowsof crowned rollers can effectively reduce friction loss and wear,provide an increased longevity against that flaking of an edge of arolling and running trace which starts at a surface thereof, and preventthe roller's radially outer surface from flaking. TABLE 1 Results ofLongevity Test Presence/Absence Estimation of of Flaking Specificationof Roller Longevity or Site of Flaking Torque Geometry: No. of 10% Ratioof Roller Motor's Sample crowned/ Rows of Longevity 10% Bearing CurrentClassification No. straight Rollers (h) Longevity Ring RollerConsumption (A) Comparative 1 straight 1 11 1.0 edge of outer 7.0Example inner surface surface Comparative 2 straight 2 28 2.5 portionouter 4.3 Example adjacent surface to of radially radially inner outerrow roller Comparative 3 crowned 1 54 4.9 edge of none 5.7 Example innersurface Example of the 4 crowned 2 83 7.5 portion none 3.9 presentinvention adjacent to radially inner row

Second Embodiment

Two rows of crowned rollers varying in effective length and in geometryat their end surfaces were used in a test. The test provided a result,as shown in Table 2. A cage having pockets arranged radially in two rowsor more (sample 5) prevented interference between rollers, enhanced thecage's ability to hold the rollers, reduced the rollers' skew to reducefriction loss and wear to provide an increased longevity againstflaking, providing a ratio of 1.5 by the 10% longevity.

It is apparent from the test result that for two or more rows of rollerswith a radially inner row of crowned rollers and a radially outer row ofrollers less crowned than the inner rollers (sample 6) and two or morerows of rollers with a radially outer row alone formed of straightrollers (sample 7), reducing the contact surface pressure of theradially outer rollers, as compared to that of the radially innerrollers, can prevent flaking of the rollers' radially outer surfaces anda roller bearing ring's radially outer side to provide an increasedlongevity.

For a radially inner row of rollers increased in length and a radiallyouter row of rollers smaller in length than the radially inner rollers(sample 8, with the inner rollers shorter and the outer rollers longerthan those in two rows of other embodiments), increasing the loadcapacity of the radially outer row, as compared to that the radiallyinner row, prevented flaking of the rollers' radially outer surfaces andthe roller bearing ring's radially outer side and also reduced adifferential slip of the radially inner rollers, and significantlyexhibited an enhanced longevity (a ratio of 1.5 by the 10% longevity).The test results reveal that it is similarly effective to provideradially outer, circumferential rollers (or pockets) equal or greater innumber to or than radially inner rollers to increase a load capacity ofthe radially outer row (FIG. 2C).

Furthermore, rollers that have a spherical surface (samples 9 and 10)prevented the rollers having their end surfaces interfering with eachother and the rollers' end surfaces and the cage's pockets interferingwith each other and thus remarkably presented an increased longevity(ratios of 1.3 and 1.6 by the 10% longevity) As a motor consumed acurrent smaller than in samples 4 and 5, it is also apparent that theyare effective in reducing friction of the roller's skew and reducing thebearing's torque. Furthermore in samples 4-10 of the present invention amotor tended to consume a smaller current than in the comparativeexample, which indicates that the bearing's internal friction loss issmall and the bearing's torque is reduced. TABLE 2 Results of LongevityTest Presence/Absence of Flaking or Site Specification of Roller Ratioof Flaking Torque Radially Inner Roller Radially Outer Roller No. of inMotor's Length Geometry: Length Geometry: Geometry Rows of 10% RadiallyRadially Current Classi- Sample of Roller crowned/ of Roller crowned/ ofEnd Pockets Lon- Inner Outer Consump- fication No. (Ratio) straight(ratio) straight Surface of Cage gevity Side Side tion (A) Example of 41 crowned 1 crowned straight 1 1.0 edge of none 3.9 the present innerinvention surface Example of 5 1 crowned 1 crowned straight 2 1.5 edgeof none 3.3 the present inner invention surface Example of 6 1 crowned 1less straight 1 1.0 edge of none 4.1 the present crowned inner inventionsurface Example of 7 1 crowned 1 straight straight 1 1.0 edge of none4.3 the present inner invention surface Example of 8 1 crowned 1.2crowned straight 1 1.7 edge of none 3.4 the present inner inventionsurface Example of 9 1 crowned 1 crowned spherical 1 1.3 edge of none3.5 the present inner invention surface Example of 10 1 crowned 1crowned spherical 2 1.6 edge of none 3.2 the present inner inventionsurface

Third Embodiment

A test was conducted using a needle thrust bearing 1 formed of rollerbearing rings 4 and 5, a rolling element (or roller) 2 arranged betweenthe roller bearing rings, and a cage 3 holding and guiding rollingelement 2, as shown in FIGS. 5A and 5B. The rolling element was acrowned rolling element. Rolling contact surfaces 6 and were formedbetween roller bearing rings 4 and 5 and roller 2. The needle thrustbearing had the roller bearing ring, the rolling element and the cageconfigured as shown in Table 3. TABLE 3 Specification of Roller BearingRing No. of carbide carbide Specification Ratio Portion flaked Motor'sper unit per unit of Roller 10% in Roller Current are area Geometry No.of Longevity 10% Bearing Consump- Sample No. (mm²) (%) of Roller Rollers(h) Longevity Ring Roller tion (A) 31 6540 0.5 Straight 1 row 11 1 edgeof outer 7 (standard inner surface bearing) surface 32 6540 0.5 Straight2 rows 28 2.5 portion outer 4.3 (comparative adjacent to surface ofexample) radially radially inner row outer roller 33 6540 0.5 Crowned 1row 54 4.9 edge of none 5.7 (comparative inner example) surface 34 65400.5 Crowned 2 rows 83 7.5 portion none 3.9 (comparative adjacent toexample) radially inner row 35 (example 39452 9.64 Crowned 2 rows 22720.6 portion none 3.9 of the present adjacent to invention) radiallyinner row

As shown in Table 3, of requirements of the present invention, in (a1)the density of a carbide of the roller bearing ring and the percentagein area of the carbide, (a2) difference between a single row of rollersand a plurality of rows of rollers, and (a3) the roller's geometry(crowned or not) a condition was changed in conducting a test to see howthe above requirements affect longevity, resistant to flaking. Morespecifically the present inventors considered that by setting the aboverequirements within the range of the present invention, a roller canhave an end surface free of a significant differential slip and asignificant stress concentration therearound so that friction loss andwear can be reduced and the roller bearing ring's edge can have a longlongevity, resistant to flaking, after the roller is rotated and run,and the inventors verified its effectiveness.

The longevity test was conducted using a bearing having a roller havinga diameter of 3 mm, a race having an inner diameter of 65 mm and anouter diameter of 85 mm. The test was conducted at a temperature of60-80° C. with a load of 1000 kgf at 500 rpm with a spindle oil VG2(hydraulic-pressure parameter, X: 0.101) applied as a lubricant. Thetest provided a result, as shown in Table 3, together with that of acomparative example, a needle roller thrust bearing having a standardroller and a standard cage.

In Table 3, longevity was represented by a 10% longevity of 10 testbearings. Note that the roller had a length of 7.8 mm for a single rowand 3.8 mm (by two) for two rows and it was crowned (or dropped) by 5 to15 micrometers.

As shown in Table 3, if sample 31, which corresponds to a standardbearing, had a longevity of 1, sample 32, having two rows of rollers,had a longevity 2.5 times sample 31. Furthermore, sample 33, having asingle row of crowned rollers, had a longevity 4.9 times sample 31.Sample 34, having two rows of crowned rollers, had a longevity 7.5 timessample 31. Samples 31-34 were all comparative examples and their ringsurfaces did not contain carbide having a density or an areal percentagefalling within the range of the present invention.

As compared with the above comparative examples, two rows of crownedrollers held in a cage and a roller bearing ring containing carbidehaving a density and an areal percentage falling within the range of thepresent invention, i.e., sample 35 of an example of the presentinvention had as much a longevity as 20.6 times greater sample 31,remarkably longer than samples 32-34, with the rollers completely freeof flaking. Furthermore in the test a motor consumed a smallest currentof 3.9 A for sample 35, which value is equivalent to that of sample 34and the smaller than the other samples and it also indicates thatfriction loss is small.

From the above results it has been found that a needle thrust bearinghaving two or more rows of crowned rollers held in a cage and a rollerbearing ring of the present invention can effectively reduce frictionloss and wear, provide a significantly increased longevity, resistant tothat flaking of an edge which starts at a surface thereof after theroller rotates and runs, and prevent the rollers' radially outer surfacefrom flaking.

Combining the crowned roller and the roller bearing ring having asurface layer with a high density of carbide distributed therein, canprovide a remarkable resistance to surface-flaking that significantlyexceeds an effect achieved by separately adding these requirements to aconventional needle thrust bearing and then adding them together. Inother words, resistance to surface-flaking can significantly be enhancedby synergism of a roller that is crowned and a high density of carbidedistributed in a surface layer of a roller bearing ring.

Fourth Embodiment

A test was conducted on a needle thrust bearing of a fourth embodimentusing two rows of cage-held crowned rollers varying in length having anend surface varying in geometry and a roller bearing ring. As shown inTable 4, of requirements of the present invention, how (b1) the lengthof the radially outer rollers of the rollers in the two rows, (b2)crowning, (b3) the structure of the pocket of the cage, and (b4) thegeometry of the roller's end surface affected longevity resistant toflaking, was examined. TABLE 4 Radially Inner Radially Outer Motor'sRoller Roller Geometry Ratio in Portion Flaked Current Roller RatioPresence/ Ratio Presence/ of End No. of 10% Radially Radially Consump-Sample Bearing in Absence of in Absence of Surface of pockets Longev-Inner Outer tion no. Ring Length Crowning Length Crowning Roller of cageity Side Side (A) 35 Roller 1 Present, 1 Present, Straight 1 1 edge ofnone 3.9 Bearing standard standard inner Ring of surface 36 Example 1Present, 1 Present, Straight 2 1.5 edge of none 3.3 of the standardstandard inner Present surface 37 Invent- 1 Present, 1 Present, Straight1 1 edge of none 4.1 tion standard small inner surface 38 1 Present, 1Absent, Straight 1 1 edge of none 4.3 standard straight inner surface 391 Present, 1.2 Present, Straight 1 1.7 edge of none 3.4 standardstandard inner surface 40 1 Present, 1 Present, R 1 1.3 edge of none 3.5standard standard inner surface 41 1 Present, 1 Present, R 2 1.6 edge ofnone 3.2 standard standard inner surface

FIG. 2A illustrates the needle thrust bearing of sample 35. FIG. 2A is aplan view with an upper roller bearing ring removed and FIG. 2B is across section of FIG. 2C. Sample 35 provided a single row of pocketseach accommodating two rollers arranged to have an axial directionradially.

FIG. 2B illustrates the needle thrust bearing of sample 36. FIG. 2A is aplan view with an upper roller bearing ring removed and FIG. 2B is across section of FIG. 2A. According to Table 4, sample 36, with a cagehaving two rows of pockets, allows rollers to be held further by thecage to reduce the rollers' skew and friction loss and wear and thusprovide a longevity 1.5 times sample 35 against flaking.

More than two rows of rollers with radially inner rollers crowned andradially outer rollers less crowned than the radially inner rollers, asprovided in sample 37, provided a longevity against flaking that isequivalent to that of sample 35 of an example of the present invention.Furthermore, sample 37 without radially outer rollers crowned, i.e.,sample 38 with straight rollers was also equivalent in longevity tosample 35. In samples 37 and 37, increasing a load capacity of theradially outer side effectively prevented flaking of the rollers'radially outer surfaces and the roller bearing ring's radially outerside.

Setting radially outer rollers to be 1.2 times radially inner rollers inlength, as provided in sample 39, with the radially inner and outerrollers smaller and larger, respectively, in length than two, inner andouter rows of rollers equal in length, as conventional, increases theradially outer side's load capacity to prevent flaking of the rollers'radially outer surfaces and the roller bearing ring's radially outerside and also reduce the radially inner rollers' differential slip toprovide a longevity 1.7 times sample 35. In particular, sample 39,having radially inner rollers reduced in length, reduced the radiallyinner rollers' differential slip to be smaller than conventional tworows of rollers and thus provided a remarkably increased longevity.

From these results it is apparent that increasing the number of rollersin a radially outer row (or the number of pockets) to be equal to ormore than that of rollers in an radially inner row can also increase theradially outer side's load capacity and thus provide a similar effect.This configuration can be implemented for example as the needle thrustbearing as shown in FIG. 2C.

Furthermore, a roller having an end surface protruding in an arc, asprovided in samples 40 and 41, prevented rollers from having their endsurfaces significantly interfering with each other and the rollers' endsurfaces and the cage's pockets from significantly interfering with eachother. Furthermore it is also apparent from Table 4 that the rollers'skew is reduced and friction torque is reduced.

Fifth Embodiment

The effectiveness of a needle thrust bearing of the present inventionwas verified in relation to the anti-wearability of its roller bearingring. Samples 35 and 31 (standard bearings) and other samples 34 and 32were rotated under the same conditions as Table 3 for a determinedperiod (of 10 hours). Note that sample 35 had two rows of crownedrollers held in a cage and a roller bearing ring having a surface layercontaining at least to a depth of 0.1 mm, as measured from its surface,a carbide having a grain of 0.6 μm or more in size such that 39,452 ofsuch grains were precipitated for each unitary area of 1 mm² with anareal percentage of 9.64%. After rotating and running, maximalwearability (in depth) was measured, as shown in Table 5. TABLE 5Specification of Roller Bearing Ring No. of carbide per % of carbideSpecification of Roller Maximum unit area per unit Geometry of No. ofRows Wearabilty Sample no. (mm²) area Roller of Rollers (μm) 31 6540 0.5Straight 1 row 1.5 (standard bearing) 34 6540 0.5 Crowned 2 rows 0.7(comparative example) 42 39452 9.64 Straight 1 row 0.1 (comparativeexample) 35 39452 9.64 Crowned 2 rows 0.05 (example of the presentinvention)

As measured at an end of the roller bearing ring after rotation andrunning, samples 31, 34 and 42 provided maximal wearabilities of 1.5 μm,0.7 μm, and 0.1 μm, respectively, whereas sample 35 achieved asignificantly enhanced anti-wearability, providing a maximal wearabilityof 0.05 μm.

Although the present invention has been described and illustrated indetail, it is clearly understood that the same is by way of illustrationand example only and is not to be taken by way of limitation. Forexample, most widely, the present invention is applicable to any needlethrust bearing that includes a single row of crowned rollers and aroller bearing ring having a surface layer with a prescribed, highdensity of carbide distributed therein. The spirit and scope of thepresent invention being limited only by the terms of the appendedclaims.

1-20. (canceled)
 21. A method of producing a roller thrust bearingsandwiching between inner and outer rings a plurality of rows of rollersheld in a cage to receive a thrust load exerted between said inner andouter rings, comprising the steps of: forming said cage to have a pocketaccommodating said plurality of rows of rollers; and crowing all of saidrollers of said plurality of rows and arranging said rollers in saidpocket to provide reduced differential slip, although contact surfacepressure attributed to said thrust load exerted on said plurality ofrows of rollers by said inner and outer rings is larger than when saidpocket has arranged therein a plurality of rollers all having a straightprofile
 22. The method of claim 21, wherein said plurality of rows aretwo rows and said pocket has two crowned rollers arranged therein.